The present invention relates to oleochemical raw materials and provides a novel process for obtaining fatty acids with improved quality.
Different approaches to the distillation of fatty acids have been reported in literature and patents and have successfully been practiced in the production scale. The aims of the existing technologies is to separate different kinds of by-products from the fatty acids, namely                partial glycerides from incomplete fat splitting and other highboilers,        metals (e.g. from catalysts),        color bodies and        odor substances.        
In these prior art approaches, highboilers and metals are simply separated by distilling the fatty acids overhead, leaving the higher boiling material as the bottom product. In order to decrease the amount of residue and reduce heat stress for the acids during 25 distillation, technologies have been developed to do the distillation in several steps at different heating temperatures [e.g. U.S. Pat. No. 4,680,092]. To further improve the highboiler separation, the use of structured column packings as a rectifying section has been introduced [U.S. Pat. No. 4,595,461, WO 96/40851 A1].
The separation of color bodies is more complicated as there may be low as well as high boiling substances present. Moreover, there may be color bodies present in the distillation, which were already contained in the raw acid as well as color bodies which are generated due to the heat stress during the distillation. The separation of low boiling color bodies can either be done as a precut in a separate unit before the final distillation or in a single unit operating as a side stream column [DE 19531806 A1]. High boiling color bodies can be separated together with other high boiling products via simple overhead distillation or by the use of structured column packings as a rectifying section.
The situation for odor substances is the same as for low boiling color bodies: They may be either present in the raw material or generated due to heat stress during the distillation. As a result, the separation of odor substances is achieved along with the separation of the lower boiling color bodies either in a precut column or in a sidestream column.
All technologies described above for the separation of low boiling color bodies and odor substances suffer from the disadvantage that they are limited to either an efficient separation of side products that were already present in the raw acid or of those which are generated during the overhead distillation. None of the existing technologies is suitable to solve both separation problems at the same time. While with the precut column technology existing lowboilers can be separated efficiently, additional low-boilers generated during the final distillation are not separated but end up in the final product. The sidestream column is in principle able to separate both existing and generated lowboilers, but with a limited efficiency, because the lowboilers pass through the sidestream on their way up the column and inevitably part of them end up in the sidestream distillate. It was also stated that some of the low boiling color bodies already present in the raw acid form high boiling condensation products in the condenser of the sidestream column. The highboilers formed are passing through the sidestream on their way down the column and part of them will end up in the sidestream distillate too. A slightly improved technology is the sidestream column with a flash unit in front. Part of the lowboilers already present in the raw material are stripped off in a flash unit and are fed to the sidestream column in a section above the side stream withdrawal. As a result, the lowboilers are bypassed around the sidestream point so that they cannot contaminate the sidestream distillate. However this technology still has three disadvantages:                The concentration of the lowboilers in the flash unit is not very efficient because it provides only one single separation stage.        Moreover some of the higher boiling components may be flashed off with the precut, are fed to the point above the sidestream and part of them may end up in the sidestream distillate.        If the above stated effect of highboiler formation as condensation products from the color bodies present in the feed material is valid, this technology doesn't prevent the condensation products from ending up in the sidestream distillate.        
Therefore, the problem underlying the present invention has been to develop a new process for obtaining fatty acids with improved color, odor and heat stability predominantly based on crude olein fractions, which is free from the disadvantages cited above.